The first truly Earth-like alien planet is likely to be spotted next year, scientists claim, saying the epic discovery would cause humanity to reassess its place in the universe.

Astronomers may have found a number of exoplanets over the last few years that share one or two key traits with our planet - such as size or inferred surface temperature ¿ they have yet to bag a bona fide "alien Earth".

However, scientists say that should change in 2013, 'Space.com' reported.

"I'm very positive that the first Earth twin will be discovered next year," said Abel Mendez, who runs the Planetary Habitability Laboratory at the University of PuertoRico at Arecibo.

Astronomers discovered the first exoplanet orbiting a Sun-like star in 1995. Since then, they have spotted more than 800 worlds beyond our own solar system, and many more candidates await confirmation by follow-up observations.

NASA's prolific Kepler Space Telescope, for instance, has flagged more than 2,300 potential planets since its March 2009 launch. Only 100 or so have been confirmed to date, but mission scientists estimate that at least 80 per cent will end up being the real deal.

"The first planet with a measured size, orbit and incident stellar flux that is suitable for life is likely to be announced in 2013," said Geoff Marcy, a veteran planet hunter at the University of California, Berkeley, and a member of the Kepler team.

Mendez and Marcy both think this watershed find will be made by Kepler, which spots planets by flagging the telltale brightness dips caused when they pass in front of their parent stars from the instrument's perspective.

Kepler needs to witness three of these "transits" to detect a planet, so its early discoveries were tilted toward close-orbiting worlds (which transit more frequently). Over time, the telescope has been spotting more and more distantly orbiting planets - including some in the habitable zone.

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